Heat, nitrogen levels may be cause of dead fish on the North Shore


Beachgoers may have noticed dead fish coming in on the tide at beaches along the North Shore. While visually disturbing, experts say this is a largely natural occurrence, however, humanity’s environmental impact is likely connected to the phenomenon.
Rising summer temperatures cause oxygen levels in the Long Island Sound to drop. Warm water does not hold oxygen as effectively as cold water, leading to a deficiency in oxygen levels, and in some cases even an absence of oxygen in small parts of the Sound.
For fish like Atlantic menhaden, most commonly known on Long Island as bunker, this situation can prove deadly. Bunker travel in large, slow-moving, and tightly packed schools. When panicked by a predator, they swim vigorously in large groups, rapidly depleting the already low oxygen levels in the water, causing them to asphyxiate.
“The heat has been a major factor in reducing the amount of dissolved oxygen in the water, and this year, even very recently, the heat’s been pretty bad,” Martha Braun, the Coalition to Save Hempstead Harbor’s environmental monitor explained. “So that’s been a big contributor to us seeing them around Sea Cliff Beach, and we’ve seen them in other parts of the harbor too.”
These large numbers of dead fish are commonly referred to as “fish kills,” which refers to a large number of deaths amongst the fish populations in a localized area. This phenomenon is not necessarily new. Over the last decade more and more fish kills have been reported in the area. The dead fish provide some benefit to the aquatic ecology of the region, as food for seagulls and other scavengers.

Heather Johnson, executive director of the Friends of the Bay, said they’ve seen higher numbers of fish kills over the last few years than they would normally expect. Equally startling is the larger area in which fish kills are being reported; whereas normally these aquatic occurrences have been confined to smaller areas, now they are seeing dead fish all along the beaches and harbors in the Sound.
“What’s alarming is the fact that this is happening all over, you know, even right in the middle of the bay,” Johnson said. “It isn’t like it’s just in the places you’d expect to see it when the temperature gets higher.”
While the increased heat caused by climate change has certainly had an impact, some experts maintain that the human effect on the Sound has direct effect on the larger numbers of dead bunker. Peter Linderoth, director of water quality at Save the Sound, attributes increased nitrogen levels in the water as a contributing factor.
According to Linderoth, Long Island’s reliance on outdated cesspools and septic tanks has had a rising effect on nitrogen levels in the water. In addition, storm water runoff can also bring nitrogen from residential lawn fertilizer into the Sound as well, further reducing oxygen levels in the water.
While higher nitrogen levels have become an unfortunate trend in the Sound, there are some ways to combat it. The planting of oyster reefs in the harbor by environmental groups like Seatuck and local governments like the Town of Oyster Bay are excellent ways to restore more normal oxygen levels, as shellfish are natural nitrogen filters.
Linderoth maintains that the most effective way to counter the issue is by reevaluating the way Long Island deals with its waste and runoff.
“Generally speaking, the best way we can really control it is through our wastewater treatments,” he said. “The more advanced treatment for nitrogen, the better, and that leads to the lower concentration of nitrogen and a lower load of nitrogen in the water.